A poplar solution

To Meet future demand for biofuels, the U.S. will need to produce corn and a variety of other feedstocks, including cellulosic material, such as poplars.

Producing poplars for cellulosic ethanol plants is not that far out of reach. In fact, Washington State University (WSU) is working with private industry partners to assess the economic feasibility of producing poplar biomass for the production of cellulosic ethanol. This work is being conducted through a grant from the Sun Grant Initiative at the Western Regional Center based at WSU.

The university is partnering with GreenWood Resources, Portland, OR, a hybrid poplar company. The company has established the GreenWood Tree Farm Fund, a private equity fund, to acquire, develop and manage fast-growing-tree farms.

Business models

Brian Stanton, managing director of tree improvement at GreenWood Resources, thinks that, in the future, integrated companies could produce both feedstocks and ethanol.

Another likely business model would be a growers' cooperative where a number of farmers would share equipment, expertise, transportation and storage facilities to provide sufficient amounts of feedstock to such plants.

Just what would it take to grow and sell poplars as an ethanol feedstock? It would depend on the size of the ethanol facility and whether that facility could convert a variety of feedstocks (for example, woody biomass in addition to ag residues or municipal waste) into fuel.

Stanton provides an example. An ethanol facility that produced 40 million gallons of cellulosic ethanol per year would need about 400,000 tons of feedstock annually to keep it running. If the ethanol facility counted on poplars to provide 30% of its annual feedstock needs, it would require the production of about 4,000 acres of poplar trees each year, assuming that each acre yielded about 30 dry tons of biomass per year following a three-year growing cycle.

Poplar advantages

Currently, GreenWood Resources is growing poplars on a 15-year rotation for sawmills. The portions of the trees that cannot be used for producing boards are chipped into feedstock for co-generation energy plants or liquid fuels conversion.

Stanton lists several advantages to poplar as a cellulosic ethanol feedstock:

Hybrid poplars are fast growing and can produce high yields of biomass annually. (WSU reports that, by applying agricultural methods, poplar plantations produce 70- to 80-ft. trees with an 8- to 10-in. diameter in six to eight years.)

Poplars can be harvested year round.

The transportation cost for poplar trees is lower than that for forest thinnings because poplars generally are located closer to fuel facilities.

Poplars have little to no need for fertilizers.

Poplars have a perennial habit, and feedstock can be stored on the stump.

Scientists in both the public and private sectors are researching how to alter lignin, the glue-like polymer in the cell walls of plants that provides strength to fibers. Lignin is difficult to break down for cellulose extraction and requires chemical pretreatment, which increases biomass-to-fuel conversion costs.

However, scientists at the Forest Biotechnology Group at North Carolina State University have identified genes that control lignin formation in fast-growing poplar trees and have modified the genes to reduce the lignin content of the trees by as much as 50%.